Cameras utilized in providing an image of a radioactive source are frequently composed of a scintillator crystal and a collimator for guiding radiant energy from the source to the scintillator crystal. An array of photomultipliers is positioned on the side of the scintillator crystal opposite the collimator for receiving flashes of light emitted by the crystal in response to the incident radiant energy. Typically, the measure of light energy received by each photomultiplier is obtained by integrating circuits coupled to each of the photomultipliers, the relative magnitude of these energies serving to indicate the location of each of the light flashes on the crystal. The stored energy of each of the integrating circuits is converted to a signal suitable for combining with the signals obtained from the other integrators. Weighting circuitry is utilized in the combining of these signals to provide points on a display, such as a cathode ray tube display, which correspond to the points of illumination upon the scintillation crystal. A circuit having an electronic window, frequently referred to as a pulse height analyzer, is utilized in the coupling of the signals from the photomultipliers to the display to insure that only signals having an energy within a prescribed range of energies are coupled to the display.
A problem arises in that the photomultiplier are often placed close to the scintillator crystal to maximize the resolution of the camera; however, the close spacing increases the nonlinear relationship between the locations of points on the display and corresponding locations of the light flashes on the scintillator crystal. Thus, for example, a sequence of light flashes of the scintillator crystal arranged in a straight line, being formed in response to a radioactive subject having the form of a straight line, may be bowed due to the geometric relationships between the individual flashes of light and the positions of individual ones of the photomultipliers. The angle of orientation of a flash of light as viewed from one of the photomultipliers varies nonlinearly with respect to displacement of the flash of light from the axis of the photomultiplier, the nonlinearity of this relationship becoming more pronounced with the closeness of the photomultiplier to the scintillator crystal. In addition, the intensity of the light flashes as received by an individual photomultiplier varies in a nonlinear fashion as a function of the position of the light flash from the axis of a photomultiplier. As a result, an observer of the display can detect nonlinearities in the displayed image of a subject viewed by the gamma camera.